1. Field of the Invention
The invention relates to magnetic inductive flow meters.
2. Description of the Background Art
Magnetic inductive flow meters use a measuring method that is based on Faraday's law of electromagnetic induction. The first basis for the magnetic inductive measurement of the flow velocity of fluids was reported in 1832 in a publication by Michael Faraday. Modern electronic switching technology in conjunction with alternating magnetic fields made it possible to overcome the separation of the useful signals, proportional to the flow velocity, from interference signals, which occur in electrochemical processes during the generation of the magnetic field at the electrodes used for signal decoupling. Thus, nothing seemed to stand in the way of the wide industrial use of magnetic inductive flow meters.
The measuring principle of magnetic inductive flow meters utilizes the separation of moving charges in a magnetic field. The conductive fluid to be measured flows through a tube which is made of nonmagnetic material and whose interior is electrically insulated. A magnetic field is applied from the outside by means of coils. The charge carriers present in the conductive fluid, such as ions and other charged particles, are deflected by the magnetic field: the positive charge carriers to one side and the negative charge carriers to another side. A voltage, which is detected with a measuring device, arises owing to the charge separation at measuring electrodes arranged perpendicular to the magnetic field. The value of the measured voltage is proportional to the flow velocity of the charge carriers and thereby proportional to the flow velocity of the measuring fluid. The flow volume can be determined over time by integration.
In magnetic fields generated by pure alternating voltage, induction of interference voltages occurs in the electrodes, which must be suppressed by suitable but costly filters. For this reason, the magnetic field is usually generated by a clocked direct current of alternating polarity. This assures a stable zero point and makes the measurement insensitive to effects by multiphase substances and inhomogeneities in the fluid. In this way, a usable measuring signal can also be achieved at a low conductivity.
If a measuring fluid moves through the measuring tube, according to the induction law a voltage is present at both measuring electrodes, which are arranged in the measuring tube perpendicular to the flow direction and perpendicular to the magnetic field. This voltage in the case of a symmetric flow profile and a homogeneous magnetic field is directly proportional to the average flow velocity. The inductive flow measuring method is capable of generating an electrically usable signal for further processing directly from the flow. The following equation basically applies:U=k*B*D*v 
where U=voltage, k=proportionality factor, B=magnetic field strength, D=tube diameter, and v=flow velocity.
A possible realization of a magnetic inductive flow meter is disclosed in U.S. Pat. No. 6,626,048 B1, which is incorporated herein by reference. This publication presents the physical and electronic fundamentals.
It is understood that major problems must be solved in the practical realization of a magnetic inductive flow meter.
In one respect, this is a matter of the material. The measuring tube must be amagnetic in order not to interfere with the magnetic field. The measuring tube further must be electrically insulating in order not to interfere with the picking up of the voltage with use of the electrodes. Moreover, the tube must have a food-safe material, when the liquid is a food, for example, drinking water.
These requirements can be fulfilled best when a food-safe plastic is used as the material. Nevertheless, plastics have the disadvantage of a much lower strength compared with metal. Resistance to internal pressure, however, is an essential requirement. The attempt to achieve internal pressure resistance with an increased thickness of the tube wall is not practicable, because otherwise the magnetic field would be weakened too greatly.
As mentioned above, the voltage at the measuring electrode is proportional to the magnetic field strength, provided that the magnetic field permeates the measuring channel homogeneously. U.S. Pat. No. 6,626,048 B1 disclosed a solution for a circular cylindrical measuring channel; this solution consisted of a magnetic coil with a magnetic core made of ferromagnetic electrical sheet steel and two magnetic poles coupled to the magnetic core and made of soft magnetic electrical sheet steel. Practical tests have shown, however, that satisfactory measurement results cannot be achieved with this arrangement. The reasons for this are the relatively long field lines and the high magnetic resistance in the electrical sheet steel, because the magnetic circuit is arranged around the electrodes.